1. Field of the Invention
The present invention relates to a power supply apparatus to which an alternating-current power supply is input and from which a direct-current voltage is output, and in particular, the present invention relates to a switching power supply apparatus equipped with a PFC converter that suppresses harmonic currents.
2. Description of the Related Art
There are regulations concerning harmonic currents, with respect to the power capacity of electrical appliances connected to a commercial power supply, and switching power supply apparatuses to which a commercial power supply is input are often equipped with a power factor correction (PFC) converter in order to comply with such regulations.
A general switching power supply apparatus, which receives a commercial alternating-current power supply as an input power supply, rectifies and smoothes the commercial alternating-current power supply, thereby converting the commercial alternating-current power supply into a direct-current voltage, and since the direct-current voltage is then subjected to switching by a DC-DC converter, the input current becomes discontinuous and significantly distorted from a sine wave. This is a cause of the generation of harmonic currents.
Consequently, in order to suppress such harmonic currents, a PFC converter is provided next to a full-wave rectifying circuit and upstream from a smoothing circuit based on a smoothing capacitor.
The PFC converter is defined by a chopper circuit and operates such that the waveform of the input current has the shape of a sine wave having the same phase as and a similar shape to the waveform of the input voltage. Accordingly, harmonic currents are suppressed to a certain level or less and the power factor is also improved.
Since such a PFC converter is a type of boost converter, the conversion efficiency of the PFC converter itself affects the power conversion efficiency of the power supply apparatus as a whole. If the output voltage of the
PFC converter is insufficiently boosted, the power conversion efficiency decreases.
Independent of a downstream DC-DC converter circuit and the load state of the downstream DC-DC converter circuit, if the output voltage of the PFC converter (that is, the input voltage of the downstream DC-DC converter) is controlled so as to be constant, in the case of a light load, an excessively high input voltage is applied to the downstream DC-DC converter and the power conversion efficiency of the downstream DC-DC converter is decreased.
Japanese Unexamined Patent Application Publication No. 2001-268897 and Japanese Unexamined Patent Application Publication No. 2007-181362 disclose conventional techniques used to prevent this problem. A power supply apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2001-268897 performs control such that a PFC converter outputs a voltage that is greater than a peak value of the input voltage by a certain amount.
A power supply apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2007-181362 stops operation of a PFC converter when the load is light in accordance with feedback from the load.
Here, the configuration of a switching power supply apparatus described in Japanese Unexamined Patent Application Publication No. 2007-181362 will be described with reference to FIG. 1. The switching power supply apparatus illustrated in FIG. 1 includes a rectifier 2 that coverts an alternating-current input voltage 1 having a wide range into a direct-current voltage, a boost-type power converter 3a that supplies this rectified output to a DC-DC converter 4a, a PFC control circuit 42a that controls the PFC converter, a load detection circuit 41 that detects the state of a load, an output voltage controller 43 that controls an output voltage, and a PFC on/off circuit 44 that switches the PFC control circuit between operating and being stopped.
The PFC on/off circuit 44 compares a detection output value of the load detection circuit 41 and a determination reference value used for determining the state of the load and the PFC control circuit 42a causes the PFC converter to operate or stop on the basis of the result of the comparison.
However, with the power supply apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2001-268897, there is a problem in that the output voltage of the PFC converter is determined based on only the input voltage, which is not related to the load. With the power supply apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2007-181362, there is a problem in that a dedicated circuit for detecting and feeding back the weight of the load is required.